Method for making a high-speed reusable x-ray plate using orthorhombic lead oxide and resulting article



July 1, 1969 E. s. ANOLICK ET AL 3,453,141 RAY PLATE EAD OXIDE AND RESULTING ARTICLE METHOD FOR MAKING A HIGH-SPEED REUSABLE X USING ORTHORHOMBIC L Filed April 25, 1966 Sheet of2 F! G URE I F I G U RE 3 O 99 9 P170 D 99.5 PbO A SELENIUM EXPOSURE MILLIROENTGENS INVE N TORS:

EUGENE S. A NOL ICK JOSEPH J. MON TEL E ONE y Ma 02 8% 7 His Attorney July 1, 1969 5, ANQLICK ET AL 3,453,141

METHOD FOR MAKING A HIGH-SPEED REUSABLE X-RAY PLATE USING ORTHORHOMBIC LEAD OXIDE AND RESULTING ARTICLE I 1' Filed April 25. 1966 Sheet 4 or 2 FIGURE 2 SELENIUM PLATE SAMPLE s TOO- I50 VOLTS DIFFERENCEQ 5 SECONDS 3.5 SECONDS DIFFERENCE Q 450 VOLTS SURFACE VOLTAGE (VOLTS) loo-"- o 5 l0 I5 20 2s 30 TIME OF IRRADIATION (SECONDS) IN VEN TORS EUGENE S. ANOLICK JOSEPH J. MONTELEONE His Attorney U.S. Cl. 117--201 United States Patent 3,453,141 METHOD FOR MAKING A HIGH-SPEED REUS- ABLE X-RAY PLATE USING ORTHORHOMBIC LEAD OXIDE AND RESULTING ARTICLE Eugene S. Anolick, Louisville, Ky., and Joseph J. Monteleone, Pittsfield, Mass., assignors to General Electric Company, a corporation of New York Continuation-impart of application Ser. No. 300,245, Aug. 6, 1963. This application Apr. 25, 1966, Ser. No. 544,928

Int. Cl. B44d 1/18; G03c N00 15 Claims ABSTRACT OF THE DISCLOSURE A process for the formation of a high speed, reusable X-ray plate using orthorhombic lead monoxide in a binder as photosensitive material. The process comprises firing the lead monoxide in a dry atmosphere, mixing the lead monoxide with an insulating binder, coating the mixture over a conductive member and heat treating the so formed plate. The X-ray plate is characterized by speeds in excess of ten times of that of the standard selenium plate and twice that of the resin bonded tetragonal lead monoxide plates in the X-ray range.

This application is a continuation in part of my copending U.S. patent application Ser. No. 300,245, filed Aug. 6, 1963 and now Patent No. 3,266,932.

This invention relates to an electrophotographic plate which utilizes lead monoxide as the sensitive photoconductive element. More particularly, this invention relates to an electrophotographic plate or photoreceptor which is highly responsive to irradiation, particularly in the X- ray range. Specifically, this invention relates to an electrophotographic plate comprising a relatively conductive backing member having on at least one surface thereof a coating of finely divided, high chemical purity orthorhombic lead monoxide dispersed in a high electrical resistant binder.

In the xerographic process as described in U.S. Patent No. 2,297,691, to C. F. Carlson, a base material of relatively high electrical conductivity such as metal, treated paper, etc., having a photoconductive insulating surface thereon is electrostatically charged in the dark. The charged coating is then exposed to a source of irradiation. The charge leaks oil rapidly to the base plate in proportion to the intensity of the irradiation to which any given area is exposed. After such exposure, the coating is contacted with electrostatically charged particles in the dark. These particles adhere to the areas where the electrostatic charges remain forming a powder image corresponding to the electrostatic image. The powder image can then be transferred to a sheet of transfer material resulting in a positive or negative print as the case may be having excellent detail and quality. Alternatively, where the base plate is relatively inexpensive, as of paper, it may be desirable to fix the powder image directly on the plate itself.

The xerographic process is operable in the infra red, actinic visible, ultraviolet and X-ray ranges. For many of these applications, the limiting factor is the sensitivity of the particular photoconductive insulating surface to the particular source of irradiation. For medical X-ray purposes, it is necessary that the photoconductive material be highly sensitive to X-ray irradiation as exposure of the human body to X-ray irradiation for prolonged periods of time can be extremely injurious to health. Therefore, in the xerographic processes, and in particular, the xeroradiographic processes, there has been a constant search for more sensitive photoconductive insulating materials.

Those photoconductive insulating materials heretofore known in the art include such materials as anthracene, sulfur, selenium, etc., and various mixtures of these materials such as sulfur with selenium, etc. The sensitivity of the prior art photoconductive insulating materials have been largely limited to the green, blue or near ultraviolet ranges and have a severe limitation of being only slightly light and X-ray sensitive. The first major breakthrough in the art of xerography was the discovery of the photoconductive insulating properties of highly purified vitreous selenium. This material has become the standard in commercial xerography. The photographic speed of this material is many times that of the prior art photoconductive insulating materials. However, vitreous selenium sulfers from two serious defects. The first difiiculty is that its spectral response is very largely limited to the blue, or near ultraviolet range. Secondly, the preparation of uniform films of vitreous selenium has required highly involved and critical processes, particularly vacuum evaporation. Furthermore, vitreous selenium by its nature requires a relatively firm and uniform support such as a continuous plastic or metal base. This, together with the high cost of selenium itself has rendered impractical the development of a disposable xerographic plate, such as a paper base plate using this material. The next advance in xerographic plates occurred with the discovery of the binder plate as described in U.S. Patent 2,663,636 to A. E. Middleton. As described therein, it was found that a xerographic sensitive member could be prepared by intimately mixing and grinding together a photoconductive insulating material and a high electrical resistant binder. Such a mixture is suitable as the photoconductive insulating layer in the xerographic plate and may be coatedon any suitable support material offering a relatively low electrical resistance, such as metal, treated paper, suitable plastics, conductively coated glass, textiles, etc.

Xerographie plates having photoconductive insulating layers prepared in accordance with the teachings of Middleton have generally been characterized by relatively low photographic speed and relatively limited spectral response for any particular pigment. In general, the sensitivity of unmodified binder'plates to photo flood illumination compared to a commercial vitreous selenium plate ranges from about 10% to a fraction of 1% of the sensitivity of the vitreous selenium plate. No single component photoconductor either in a binder plate or in a vitreous xerographic plate has been found which even remotely approaches the spectral sensitivity of vitreous selenium. Accordingly, of the regular xerographic processes, vitreous selenium has remained the material of choice.

The next advance in the art of xerographic plates was reported in U.S. Patent No. 3,008,825 of Warren G. Van Dorn, issued Nov. 14, 1961. As described therein, a xerographic plate is prepared by intimately mixing and grinding together tetragonal lead monoxide in a high electrical resistant binder. The use of lead oxides as photoconducting materials is well known in the art, and has been reported as early as 1939 in a patent to H. M. Lewis et al., U.S. Patent No. 2,169,840. However, as reported by Van Dora, the crystalline forms of lead monoxide, other than the tetragonal form, were inoperative in xerographic processes or showed little light sensitivity. Therefore, it was reported by Van Dorn that only the tetragonal form of the oxide of lead is the truly outstanding light sensitive photoconductive material. In fact, Van Dorn compares tetragonal lead monoxide with orthorhombic lead monoxide and reported that C.P. grade orthorhombic lead monoxide showed no light sensitivity, C.P. grade tetragonal lead monoxide largely, but not completely converted to orthorhombic lead monoxide had a light sensitivity of 0.76; and C.P. grade yellow orthorhombic lead monoxide completely converted to red tetragonal lead monoxide had a sensitivity of 4.6, all compared to a selenium plate which was used as a reference with a light sensitivity of a value of 1.

In copending US. patent application Ser. No. 300,245, noted above, there is disclosed a process for forming xeroradiographic plates from orthorhombic lead monoxide. The process comprises the steps of heat treating or firing the orthorhombic lead monoxide under conditions which prevent the formation of other oxides of lead, mixing the heat treated orthorhombic lead monoxide with an appropriate insulating binder solution to form a suspension, coating a conductive surface or backing member with the suspension, and drying the composite to obtain the electrophotographic plate. By following this procedure, it was found that electrophotographic plates could be formed having speeds from 2 to 4 times greater than the standard selenium plate known in the art.

In accordance with the present invention, it has been found that a resin bonded orthorhombic lead monoxid plate can be formed having speeds in excess of times that of a standard selenium plate and in excess of twice that of a resin bonded tetragonal lead oxide plate in the X-ray range, and at least equivalent to the prior art plates in the visible range. In addition, it has been found that if the surface of the electrophotographic plate prepared in accordance with the teachings of the present invention are treated in a manner so as to provide a hard, smooth surface, the plates are reusable in excess of 100 times resulting in very low cost for each use.

Accordingly, one object of this invention is to provide a fast, reusable electrophotographic plate highly responsive to irradiation, particularly in the X-ray range comprising a relatively conductive backing member having on at least one surface thereof a coating of finely divided orthorhombic lead monoxide dispersed in a high electrical resistance binder.

Another object of this invention is to provide an electrophotographic plate having an orthorhombic lead monoxide surface coated with a hard, smooth material.

A third object of this invention is to provide a method for the formation of a fast, reusable electrophotographic plate highly sensitive to irradiation.

Other objects and advantages of this invention will be in part apparent and in part pointed out in the description which follows.

In the drawings:

FIGURE 1 is a diagrammatic view, in section, of the electrophotographic plate in accordance with the present invention.

FIGURE 2 graphically compares decay of a charged electrophotographic plate formed from orthorhombic lead monoxide with a charged plate formed from selenium; and

FIGURE 3 graphically represents voltage in an electrophotographic plate formed from orthorhombic lead monoxide upon exposure to X-ray irradiation.

The speed of electrophotographic plate is of prime importance, particularly when the plate is used for X-ray purposes. Heretofore, a xerographic plate was not available for medical X-ray purposes because of insufficient speed and it was found necessary to use photographic film. To obtain a xerographic plate of sufficient speed, it is desirable that the speed exceed the speed of a selenium plate by a factor of at least 7 and preferably by a factor of 10. In accordance with the present invention, it has been found that the high speeds necessary for X-ray purposes are obtainable using resin bonded orthorhombic lead monoxide plates when the lead monoxide employed is substantially free of other oxides of lead and has a chemical purity of at least 99.5% and preferably, 99.9%, and in addition, the electrophotographic plate formed from the pure orthorhombic lead monoxide is heat treated to remove excess free oxygen from its surface.

Briefly stated, the process for forming electrophoto suspension;

(3) Coating a conductive backing member with said suspension to form a composite electrophotographic plate;

(4) Drying said electrophotographic plate;

(5) Heat treating the electrophotographic plate to remove residual oxygen from its surface; and in a preferred embodiment,

(6) Applying a hard, smooth coating to the orthorhombic lead monoxide surface.

A typical electrophotographic plate formed by the above procedure is represented in FIGURE 1. It should be understood that the dimensions in the drawing are enlarged for purposes of illustrating. The plate comprises a base plate 10 having coated thereon the photoconductive layer 11 consisting of a resinous binder having particles of orthorhombic lead monoxide 12 dispersed therein. In a preferred embodiment of this invention, a hard, smooth surface 13 is applied to the photoconductive layer so that the plate may be reused many times. Alternatively, a dark, opaque layer may be applied to the plate to block visible light and thus prevent decay of the surface charge on the plate after exposure.

The function of the base of backing member 10 used in preparing the plates is to provide physical support for the photoconductive insulating layer and to act as a ground thereby permiting the photoconductive insulating layer to receive an electrostatic charge in the dark and permitting the charges to migrate when exposed to irradiation. It is evident that a wide variety of materials may be used, for example, metal surfaces such as aluminum, brass, stainless steel, copper, nickel, zinc, etc.; conductively coated glass such as tin or induim-oxide coated glass, aluminum coated glass, etc.; similar coatings on plastics substrates as on polyethylene terephthalate, cellulose acetate, polystyrene, etc., or paper rendered conductive as by the inclusion of a suitable chemical therein or through conditioning in a humid atmosphere to insure the presence therein of sufficient water content to render the material conductive.

The first step in the formation of an electrophotographic plate involves heat treating the orthorhombic lead monoxide, in the form of particles having a particle size ranging between 0.25 and 10 microns, and preferably, between 0.5 and 5 microns, to remove water vapor from the surface of the lead monoxide and to convert all of the lead monoxide to the orthorhombic crystalline form.

Heat treatment may take place in either dry air or an inert atmosphere. If an inert atmosphere is used, the heat treatment temperature should range between 300 C. and 600 C. Alternatively, the lead monoxide may be fired in dry air. In this case, excess oxygen may be introduced into the powder to provide P-type conduction; that is, conduction due to the presence of empty energy states which permit electron movement. It should be noted that, if the heat treatment is carried out in air, the temperature range should be either C. to 350 C., preferably between 250 C. and 350 C. or in the range of 500 C. to 700 C., preferably between 550 C. and 650 C. The intermediate range, between 350 C. and 500 C. may not be used with an air atmosphere because of the transformation from lead monoxide to higher oxides of lead which are detrimental to the operation. It is noted that, if the range above 500 C. is used, the lead monoxide must be rapidly cooled through the prohibited range to inhibit transformation to the tetragonal form.

Following such heat treatment, other oxygen content controlling compounds may be introduced if it is desired to control the amount of excess oxygen and thereby increase or decrease the amount of P-type conduction. For example, oxidizing or reducing compounds could be used to respectively increase or decrease the amount of oxygen. If such conduction is not desired in the finished plate, an inert atmosphere may be provided throughout the heat treatment so that stoichiometric ratio of lead to oxygen is maintained.

It has been found that the thus heat treated lead monoxide should be handled carefully so as to avoid transformation to the tetragonal form. For example, subsequent further working to reduce the particle size of the lead monoxide, such as by severe grinding, ball milling, etc., should be avoided as this tends to cause transformation to the red tetragonal form.

Following the heat treatment step, the orthorhombic lead monoxide is dispersed in a solution of an insulating binder. Mild mixing techniques should be used to prevent transformation of the lead monoxide to another crystalline form. Mild mixing in a Waring Blendor for 5 minutes is sufficient.

The binder in which the orthorhombic lead monoxide is dispsered is a material which is an insulator to the extent that an electrostatic charge placed on the layer is not conducted by the binder at a rate which prevents the formation and retention of an electrostatic latent image or charge thereon. The binder adheres tightly to the base material and provides an efiicient dispersing medium for the orthorhombic lead monoxide. Furthermore, the binder should not react chemically with the lead monoxide. Satisfactory binder materials for the practice of this invention include acrylic and methacrylic ester polymers, particularly polymerized butylmethacrylates; vinyl polymers such as polystyrene, polyvinyl chloride, polyvinyl acetate and copolymers of these materials; alkyd resin; polyphenylene oxides, etc. In addition, mixtures of such resins with each other or with plasticizers so as to improve adhesion, flexibility, blocking, etc., of the coatings may be used.

Any manner known to those skilled in the art may be used to apply the suspension of lead monoxide and binder to the backing member. For example, the suspension may be applied by dipping, whirling, spraying, use of a doctor blade, dip roll, etc. The solvent is allowed to evaporate in air leaving a layer of the photoconductive material on the base plate.

The photoconductive layer so formed should range between /2 and mils with optimum resolution being obtained with thicknesses between 1 and 5 mils. Coatings in excess of 10 mils result in a loss of speed and dimculties in preparing and handing the materials.

Curling of the thin, base material may make it diflicult to achieve a uniform coating. This may be overcome by spreading silicone oil or alcohol on a flat, glass plate and then flattening the foil in the oil. The coating step may then be readily performed.

The weight ratio of lead monoxide to resin in the photoconductive coating may vary between 1 to 1 and 16 to 1. In a coating having a high concentration of lead monoxide, it was found that uniform wetting of the base material may be facilitated by first applying a thin layer of a vinyl acetal material in chloroform solution (5% by weight) to the base plate. The lead monoxide coating is then applied.

When a smooth, hard coating is applied to the electrophotographic plate, it has been found that the plate may be reused many times. To reuse the plate, it is necessary that the hard, smooth sun-face of the plate be polished, such as by bufiing, etc. This removes the latent image and the plate is then in condition for reuse and may be reused in excess of 100 times. Any coating material that is capable of forming a hard, smooth surface may be used. Hard parafiin waxes make excellent coatings. One particularly good coating consists of a major portion of paraflin wax with a minor portion of a wax polyethylene vinyl acetate wax modifier.

Following the step of forming the hard, smooth coating on the electrophotographic plate, it is necessary to heat treat the plate if speeds suitable for medical X-ray use are needed, i.e., speeds of at least seven times that of a commercial selenium plate. Though the effect of heat treating is not fully understood, it is believed that one function of the heat treating step is to remove excess oxygen from the surface of the plate which, if allowed to remain, might act as traps or other hindrance to current flow. Heat treating may take place either in air or an inert atmosphere, The heat treating temperature may vary between 10 and 60 minutes with lower temperatures requiring longer heat treatments. A preferred method of heat treating involves first heating the plate in air followed by passing an inert gas such as carbon dioxide over the plate while maintained at the elevated temperature.

The speed of the electrophotographic plates is greatly influenced by the chemical purity and morphology of the orthorhombic lead monoxide used. High speeds are obtained with highly pure substantially orthorhombic lead monoxide. For example, an orthorhombic lead monoxide plate having a chemical purity of 99.5% has a speed varying between 0.3 and 5 times that of the standard selenium plate depending upon the processing steps used while a plate formed from an orthorhombic lead monoxide having a chemical purity in excess of 99.9% may obtain speeds, if properly prepared, exceeding 10* times the speed of the standard selenium plates. Plates formed from an othorhombic lead monoxide having a purity of about 99.0 have speeds equivalent to selenium plates if prepared and heat treated according to the process of the present invention.

The remainder of the specification is directed to illustrations of the advantages gained through the use of orthorhombic lead monoxide plates prepared in accordance with the present invention.

EXAMPLE 1 This example illustrates a general procedure for preparing an electrophotographic plate in accordance with the teachings of this invention. A 600 gram batch of lead monoxide having a chemical purity of 99.5% is fired in a furnace for 2 hours at 575 C. The lead monoxide so fired is then stored in a vacuum desiccator maintained at C. Thereafter, the following Master Mix solution is prepared: 200 grams polyvinylbutylral is dissolved in 1000 grams of predistilled spectral grade toluene. Nitrogen is bubbled through the solution to purge the oxygen. Sixty-five grams of this solution, 15 grams of distilled toluene, and 230* grams of fired lead monoxide from the desiccator are mixed at low speeds for five minutes in a Waring Blendor, The result is a very viscous, creamy blend Next, a sheet of tempered aluminum foil, 1.5 mils thick, is placed on a flat, glass plate, moistened with isopropyl alcohol. The plate is maintained at 50 C. and a rubber roller is used to smooth the foil on the plate. The alcohol is used to maintain the foil fiat. The top surface of the foil was cleaned with alcohol and ketone solvents to prepare it for coating.

The lead oxide suspension is applied to the foil by hand. The functional part of the applicator is a diameter stainless steel cylinder, 10" long with the middle 8" accurately undercut 20 mils. When a quantity of the suspension is spread on the foil by drawing the applicator horizontally, a film 8" Wide and up to 12" long is formed. The dry thickness of the coating is accurate and is about 5 mils, depending upon the concentration of the formulation.

The film is air dried, and at this point is a light yellow in color and is quite flexible. It is then further heat treated in air at C. for 30 minutes. The result is a flexible,

light tan coating which can be cleaned with a suitable solvent.

EXAMPLE 2 This example is designed to illustrate the effect of severely working orthorhombic lead monoxide on the relative speed of an electrophotographic plate formed therefrom.

In general, the electrophotographic plates used in this example were prepared in accordance with the procedure set forth in Example 1 above. However, the mixtures of lead monoxide, binder and solvent were subjected to severe ball milling following the step of mild mixing in the Waring Blendor. The length of time that each mixture was subjected to ball milling is set forth in Table I below.

After the plates were prepared, they were charged by subjecting them to irradiation from below while passing under a vibrating probe. The apparatus was contained in a lead-lined box for operator protection and for maintaining the film in the dark since the charged plate was sensitized and would decay in the presence of light as well as X-rays.

Comparative speed of the plates can be determined using the procedure set forth in US. Patent No. 3,008,825 of Van Dorn. Using this procedure, speed is determined by the time necessary for surface charge of a charged plate to decay to /2 of its initial value in accordance with the following relationship:

Speed: I( 1/2D) i/2x) wherein:

TABLE L-EFFECT OF WORKING ON SPEED OF ELECTRO- PHOTO GRAPHIC PLATES Tiqx Tm) Comparative Ball Mill Time (hrs): Sec. See Speed From the above, it is evident that ball milling decreases comparative speed. As the period of ball milling increased, more and more red crystals were detected in the mixture indicating that ball milling caused the conversion of the orthorhombic form of lead monoxide to the tetragonal form.

EXAMPLE 3 In this example, an electrophotographic plate prepared in accordance with the procedure set forth in Example 1, was waxed on its surface to provide a smooth, easily cleaned surface. Ordinary car wax was used for this purpose. The toner powder images on the surface of the plate so waxed were readily erased by bufiing the plate with a cloth and the latent image was erased by the flash of an infra red lamp. The plate was capable of reuse times with little or no deterioration of the image. Speed remained constant throughout. Dark adaptation after infra red flash was useful, between exposure, to maintain image contrast.

EXAMPLE 4 This example shows the effect of the ratio of lead monoxide to resin on the relative speed of an electrophotographic plate formed from orthohombic lead monoxide. Four samples, S to 8.; were prepared, in weight ratios of lead monoxide to resin of, respectively, 1:1, 2:1, 4:1 and 8:1 using orthorhombic lead monoxide obtained from the Evans Lead Corporation and heat treated at 300 C. for one hour in air. The particle size range of the lead monoxide was 0.5 to 5 microns, the main portion being between 0.5 to 2 microns. A spectrographic analysis revealed the following maximum limits of quantities of impurities in the lead monoxide obtained from the Evans Lead Corporation:

Percent Bi 0.03 Sn 0.0001 Fe 0.0005 Zn 0.0005 Au 0.0002 Te 0.001

Cr 0.0001 Cu 0.0006 As 0.0006 Mo 0.0001 Mn 0.00005 Pt 0.0001 Tl 0.0001 Al 0.0005 Ag 0.001 Sb 0.001 Cd 0.0008

Co 0.0001 Ti 0.0001 Mg 0.0002

The remainder of the preparatory steps were as described in Example 1 except that the plate formed was not heat treated.

The following table indicates the speeds of several samples of lead monoxide plates in accordance with the present invention. In this example, the speeds were measured relative to that of selenium which was considered to be one.

TABLE II Sample Relative speed 8 .35 S .5 S 1.2 8.; 1.7

It is noted that the samples 5 -5 in the above table are arranged in order of increasing weight ratio of lead monoxide to resin. It therefore appears from the above table that lead monoxide plates prepared in accordance with the above described method and prepared with a sufliciently high ratio of lead monoxide present in the plate will provide substantially higher' speed than the standard selenium plate previously known. These figures represent averages over several regions of the sample. In one instance, a speed approximately ten times that of selenium was obtained.

EXAMPLE 5 This example illustrates the decay characteristics of a charged electrophotographic plate prepared in accordance with the present invention.

An electrophotographic plate was prepared from a reagent grade orthorhombic lead monoxide obtained from the Fisher Company and having the following analysis.

The powdered lead monoxide was mixed with 2% lead carbonate and the mixture was fired in air at 350 C. for one hour, the remainder of the preparatory steps being as described in Example 4.

The so formed plate was charged by subjecting it to irradiation from below while passing under a vibrating probe. For purposes of comparison, a commercial selenium xerographic plate was charged in the same manner.

The advantage of the lead monoxide plates over the selenium plates is illustrated by FIGURE 2, which is a plot of the decay of surface voltage under irradiation by an X-ray beam. It can be seen from this graph that while after a long period of time such as 20-30 seconds, the lead monoxide plate and the selenium plate show approximately the same discharge, in time intervals less than 20 seconds the sample prepared in accordance with the present invention is significantly faster than the selenium plate previously used. For example, after five seconds, of irradiation, the surface voltage of the selenium plate had only dropped to approximately 550 volts while the surface voltage of the lead monoxide plate had dropped to 400 volts. The selenium plate took 3.5 seconds longer to reach 450 volts, a useful value for the powder cloud technique, than did the lead monoxide plate. This decrease in the exposure time can be very significant in the case of relatively delicate samples. Thus, X-ray photographs of equal or better quality than the X-ray photographs taken using the previously known selenium plate can be made with a substantial reduction in the length of time of radiation by the X-ray beam. This is obviously advantageous in terms of reduced time necessary to operate the beam and also a reduction in the time during which any individual or object is subjected to irradiation.

EXAMPLE 6 This example illustrates the relationship between plate speed and purity of orthorhombic lead monoxide. Two electrophotographic plates were prepared in accordance with the procedures set forth in Example 1, however, the plate was heat treated 60 minutes at 150 C. in carbon dioxide after its formation. The first plate used an orthorhombic lead monoxide having a purity in excess of 99.998%. The second plate was formed from OR grade orthorhombic lead monoxide found to be 99.5% pure. The plates were charged using the procedure of Example 4 and exposed to differing levels of irradiation. FIGURE 3 represents voltage drop on the plates surface upon exposure to various pulses of irradiation 5 seconds after exposure. For purposes of comparison, an industrial selenium plate was also charged and exposed.

From FIGURE 3, it can be seen that the high purity orthorhombic lead monoxide is more sensitive than the plate formed from C.P. grade lead monoxide and far more sensitive than the selenium plate. This is evident from the large voltage drop in the high purity lead monoxide plate with comparatively low X-ray pulse.

From FIGURE 3, it is possible to compare speed of the two lead monoxide plates relative to the industrial selenium plate. Correcting all initial voltage to 700 volts. Speed is determined using the following formula:

S X 1/28 X 1/21.

wherein Xl/zL is the X-ray dose necessary to cause plate voltage to decay to /2 its original value in time T (5 seconds) for the lead monoxide plate; and X is the X-ray dose necessary to cause plate voltage to decay to /2 its original value in time T (5 seconds) for the selenium plate.

Based upon the above, the following results were found:

TABLE IIL-RELATIVE SPEED Exposure reach half voltage Speed milli- X 5] Material: roentgens Xm.

Selenium 1 99.5% PbO-.- 28- 5+ 99.998% PbO. 17

EXAMPLE 7 This example is designed to illustrate the necessity of properly heat treating the lead monoxide so as to obtain an orthorhombic lead monoxide suitable for purposes of the present invention.

Samples of GP. grade orthorhombic lead monoxide were heat treated in a manner set forth in Table III and the percentage of the orthorhombic crystalline form of the lead monoxide in the resulting sample was determined by X-ray diffraction analysis.

TABLE IV.EFFECT OF TREATMENT ON ORYSTALLINE FORM OF LEAD MONOXIDE Percent Ortho- Treatment Color rhombic (1) O.P. Grade Untreated Orange 40 (2) 1 hr. at; 350 C Pale yellow"-.- (3) 2 hr. at; 600 C Bright yellow.-. (4) 2 hrs. at 600 0. surface abraded Red 40 The effect of heat treatment, particularly at 600 C. is quite pronounced. However, Sample 4 indicates that the orthorhombic form of the lead monoxide is transformed to the tetragonal form by abrading the surface due to the mechanical working of the lead monoxide.

EXAMPLE 8 In this example, an electrophotographic plate was prepared by firing C.P. grade orthorhombic lead monoxide for two hours at 600 C. and then cooling to room temperature. Thereafter, the orthorhmobic lead monoxide was added to a solution of purified polyvinyl butyral dissolved in dry toluene and blended in a Waring Blendor for five minutes. The mixture was then coated on aluminum foil and allowed to dry. The composite was fired at C. for 30 minutes and then waxed with an automobile wax designated by the commercial name of Vista Wax. The coating of lead monoxide on the aluminum foil was found to be approximately 8 mils thick. The ratio of lead monoxide to binder was approximately 16 to 1. The plate was exposed to an X-ray source capable of 80 kvt. operation and a full sized xeroradiograph of a sheep shank at 30 milliroentgens for 1/30 second at 15 inches was obtained. The image quality was good. The image was erased and reproduced with useful detail with exposures of only 12 milliroentgens.

1 1 EXAMPLE 9 Additional electrophotographic plates were formed using the above procedures. A commercial grade orthorhombic lead monoxide having a chemical purity of about 99.5% was used. A number of subjects were photographed using an X-ray source of 125 kv. at 2 milliamperes. The plates were charged, exposed and developed in rapid sequence, there being less than 30 seconds between the start of charging and the viewing of the final image. The intensity of the source was such as to provide l2 milliroentgens at the subject in one second exposure. A human hand exposed for 1 second yielded a clear picture of the bone structure. At /2 second, a faint picture was discernible and at A second, practically no detail was evident. This compares with the fastest photofilm which yields the same information at A second as the commercial lead monoxide plate at 1 second.

EXAMPLE Electrophotographic plates were formed in accordance with the procedures set forth in Example 1. A microscope illuminator was used as a source of irridescent light. It was placed so as to illuminate the plate under a measuring probe. The illumination was at a glancing angle to the plate. For comparison purposes, a selenium plate was used as a standard. It was found that the visible light response was in direct proportion to X-ray response, i.e., the faster the X-ray response, the faster the light response. A plate having speeds of 3 times a selenium plate in an X-ray beam showed speeds equivalent to the selenium plate in visible light.

In addition, the same lead monoxide plates were charged, light exposed through a test TV pattern plate in contact with the lead monoxide surface and developed by the cascade of toner and iron filings. Clear, high contrast images were produced by this method. These images could be erased and the plates reused after dark adaption.

The development of the electrostatic images in accordance with the teachings of this invention are similar to those used in other xerographic processes. Normally, electrostatic images are made visible by bringing them in contact with a pigmented, finely divided powder which is attracted to, and held on, the electrostatic image. Particles will behave thus if they hear an electrostatic charge opposite to that of the image areas of the photoreceptor. The powder can be delivered to the electrophotographic plate by simply pouring it over the photoreceptor and blowing or brushing off the excess. For better control of development, the developer powder, or toner, is mixed with a much coarser granular material, the carrier, and the combination is cascaded across the exposed xerographic photoreceptor. The carrier material is selected so that it produces electric charge on the toner opposite in polarity to that of the image to be developed. In another form of development, a fur brush serves as a carrier by giving the toner the proper electrical charge and by transporting it across the image area. Alternatively, the image can be developed by immersing the exposed photoreceptor in insulating liquid containing a toner.

For most applications of xerography, a permanent record is the required product. With zinc oxide binder papers, the photoreceptor itself becomes the permanent support. With re-usable photoreceptor xerography the toner image must be transferred and fixed to paper or some other surface like wood, metal, ceramic, plastic or glass. The images may be transferred by placing a sheet of paper over them and applying a high potential to the paper by another application of the corona discharge used to sensitize the plate. When the paper is removed from the photoreceptor after the treatment, it carries a sizable portion of the toner image.

Alternatively, an adhesive sheet can be pressed against the toner image to pick it off the plate. Moistened gelatin coated paper can be used in this fashion; particularly with continuous toner images which usually involve a toner that does not retain enough electrical charge to permit electrostatic transfer.

The electrostatic image itself can be transferred to another insulating film where it can be developed. In such a process, the photoreceptor performs a purely electrical function and is not subject to the Wear accompanying development and cleaning.

The image is permanently afiixed to the transfer sheet by heating in an oven for a few seconds, or by passing the sheet under a radiant heat source. With adequate fixing the powder particles flow together and coalesce into a smooth layer which is firmly bonded to the transfer sheet. The small amount of residual powder remaining on the plate is readily removed by a simple wiping or brushing operation, after which the plate is ready for reuse.

The specific embodiments described herein are presented merely as examples of the various forms which this invention may take. Therefore, it is intended in the appended claims to cover all modifications and variations falling within the scope of this invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A process for forming a high speed electrophotographic plate comprising the steps of:

(a) firing a quantity of orthorhombic lead monoxide in a dry atmosphere;

(b) mixing the so formed orthorhombic lead mon xide with an insulating binder in a dry atmosphere;

(c) coating a conductive member in a dry atmosphere with said mixture to form the electrophotographic plate; and

(d) heat treating the so formed clectrophotographic plate in a dry atmosphere.

2. The process of claim 1 wherein the orthorhombic lead monoxide has a chemical purity of at least 99.0%.

3. The process of claim 1 wherein the orthorhombic lead monoxide has a chemical purity exceeding 99.9%.

4. The process of claim 1 wherein the lead monoxide is fired at a temperature ranging between C. to 350 C. or 500 C. to 700 C. in air.

5. The process of claim 1 wherein the lead monoxide is fired at a temperature of from 300 C. to 600 C. in an inert atmosphere.

6. The process of claim 1 wherein said lead monoxide is provided in powdered form, the dimensions of the particles in the powder being less than 10 microns.

7. The process of claim 1 wherein the proportion by weight of said lead monoxide to said binder ranges between 1 to 1 and 16 to l.

8. The process of claim 1 wherein the ratio of lead monoxide to binder is approximately 4 to 1.

9. The process of claim 1 wherein the electrophotographic plate is heat treated at a temperature ranging between 100 C. and 225 C. in a dry atmosphere for a period of time ranging between 10 and 60 minutes.

10. The process of claim 9 wherein the dry atmosphere is carbon dioxide.

11. The process of claim 1 including the step of applying a hard, smooth surface to the heat treated electrophotographic plate.

12. A high speed electrophotographic plate formed in accordance with the procedure set forth in claim 1.

13. A process for forming a high speed electrophotographic plate comprising the steps of:

(a) firing a quantity of orthorhombic lead monoxide having a chemical purity in excess of 99.9% in an atmosphere selected from the group consisting of air and an atmosphere inert to the lead monoxide, the temperature of said firing ranging between 100 C. to 350 C. or 500 C. to 700 C. when the atmosphere is air and 300 C. to 600 C. when the atmosphere is inert to the lead monoxide;

(b) mixing the so fired orthorhombic lead monoxide 13 with an insulating binder in a dry atmosphere in an amount of lead monoxide to binder ranging between 16 to 1 and 1 to 1;

(c) coating a conductive member in a dry atmosphere with said mixture to form the electrophotographic plate; and

(d) and heat treating the so formed electrophotographic plate in a dry atmosphere at a temperature ranging between 100 C. and 225 C. for a period of time ranging between 10 and 60 minutes.

14. The process of claim 13 including the step of applying a hard, smooth coating to the heat treated electrophotographic plate.

References Cited UNITED STATES PATENTS 5/1959 Damon et a1. 117-201 3/1967 De Haan et a1. 961 X 10 WILLIAM L. JARVIS, Primary Examiner. 

